The present invention relates to an ullage gage and more particularly relates to a gage for directly and accurately measuring the ullage of a cargo tank of a vessel such as an oil tanker.
In loading liquid cargo vessels such as oil tankers, the volume of liquid in a cargo tank is routinely determined by measuring the "ullage" of the tank after it is loaded. Ullage is generally defined as the amount by which a cargo tank falls short of being full and is directly related to the distance between the top of the cargo and the underside of the top of the tank. In most commercial tankers, the ullage measurement is made from a special hatch, i.e., ullage hatch, which sets atop the main hatch into the cargo tank. Once the ullage measurement is made, a standard set of ullage tables for that particular tanks is consulted to establish the actual volume of cargo present in the tank.
As the over-all size of a cargo tank increases, the accuracy of the ullage measurement becomes more and more important. For example, due to the extremely large tanks on some modern tankers, an error of one-half inch in the ullage measurement can mean an error of 5000 gallons or more in the actual volume of cargo in that tank. The economics involved in such an error is obvious.
In a typical cargo loading operation, liquid is normally loaded into a cargo tank at a high flow rate until the tank is filled to within 5 to 6 feet of the top of the tank which is, also, normally the underside of the main deck of the vessel. The flow rate is then substantially reduced to guard against accidental overfilling which may rupture the deck. The tank is then "topped off" by slowly filling the tank to within a few inches, e.g., 6 to 24 inches, of the deck. At this time, the final ullage measurement is taken by lowering a plumb bob measuring line through the ullage hatch. The volume of cargo in the tank is then determined from the ullage tables using this highly reliable, handmade measurement.
However, developments in recent years have complicated the use of the conventional hand ullage gage in topping out cargo tanks. For example, inert gas systems, which are designed to provide nonflammable gases under pressure in a cargo tank, have to be released before the plumb bob can be lowered through the ullage hatch into the tank. Releasing the inert gas reduces the margin of safety of the system in that there is a greater risk that air might be drawn into the cargo tank, thereby creating an explosive environment within the tank.
Further, there is now an awareness that ullage bobs lowered into cargo tanks act to concentrate the electrostatic field commonly present during tanker operations. Under certain conditions, this can cause an electrostatic discharge of sufficient intensity to ignite any explosive mixture that may be present in the tank. Therefore, the need exists for a relatively simple ullage gage which has the accuracy and reliability of a conventional ullage bob but one which does not require the opening of the tank for its use.